Evaluation of the stiffness of a car body and implementation of the RBE3 link model in Code_Aster

by Luca Dall’Olio, Alneos and P. Juillard, ArcelorMittal R&D

Performances, in terms of stiffness of a “body in white” car is an important characteristic which allows assessment of whether a vehicle being designed will (or will not) offer the requested driving comfort. In cases where stiffness is insufficient, the vehicle will produce (for example)
vibrations during the running phases, or may show a behavior too "loose" in turns or when it is loaded.

During the design of a car body, static linear simulations are therefore conducted to verify the structural strength to the given efforts, and the non-appearance of some eigen-modes beyond a given threshold frequency.

Based for the first time on Code_Aster, as well as on NECS and ALNEOS teams, such simulations were performed at the end of 2010 in ArcelorMittal Automotive Research Laboratory Applications as part of a study over a generic light utility vehicle. A comparison was then was conducted with results from Nastran equivalent models, a typically used solver
for such applications.

Figure 1 : Body in white of a light commercial vehicle

Differences observed on the displacement field in structures appear between 2 and 8%, and can partly imputed to the use of different finite element shell formulations between the two codes, as well as by the use of solid elements with full integration into the Code_Aster model, instead of under-integrated solid elements as in the Nastran model.

Figure 2 : Body in white of a light commercial vehicle, static analysis

Figure 3 : Body in white of a light commercial vehicle, the 7th eigenmode , fréquency: 29.43 (Nastran), 29.47 (Aster)

Another difference in models conception can be found in the representation of spot welds that bind the sheets of “in white” body together. Traditionally modeled by a RBE3 linkage assembly with a solid element, it was necessary to rewrite each link as a set of equations in the Code_Aster model (LIAISON_DDL keyword ) constraining the degrees of freedom of the mesh nodes lying in parts being in the immediate proximity of the welded points. The generation of tens of thousands of LIAISON_DDL keywords was handled by the finite element model converter « Vega », developed by Alneos.

As a result, an input file of over 120MB, and whose post-processing by the Aster supervisor therefore has taken a long time before even starting the model execution.

Strong of this feedback, the ALNEOS teams were then able to achieve at the end of 2011 the implementation of the RBE3 link in Code_Aster (LIAISON_RBE3 keyword in AFFE_CHAR_MECA command).

This binding model, well known in the design of assembled structures (automotive, aeronautics…), spreads the forces and moments exerted on a master node towards a set of slave nodes, taking into account the center of gravity of the set of slave nodes , the degrees of freedom activated by the user and the associated weights.

Figure 4 : Modélisation des 6000 points de soudures par des RBE3

Integrating directly into RBE3 Code_Aster helped improve performance, both as memory and computing time:

This new feature is now available since version 11.1.13.